A conventional MRI apparatus uses a nuclear magnetic resonance to image the subject that is to be imaged and is placed in a uniform static magnetic field space present in the conventional MRI apparatus. A region to be imaged is limited to the static magnetic field space. In recent years, a technique has been developed to move a table (bed) mounting thereon a subject to be imaged and image a whole body of the subject. Especially, screening of a whole human body using a MRI apparatus has gained attention.
In order to measure a large region such as a whole human body, it is desirable to realize a receiving coil capable of maintaining high sensitivity to the large region and reduce the time for imaging to set a measurement time within a time for which the subject to be measured can endure the measurement. As a technique for reducing the time for imaging in an image diagnosis process using a MRI apparatus, a technique (called parallel imaging and hereinafter referred to as parallel imaging) for using a distribution of sensitivity of a plurality of RF coils to develop an aliasing artifact is starting to be put to practical use (refer to Non-Patent Document 1). This technique uses a receiving coil composed of a plurality of sub coils to simultaneously measure signals and is capable of reducing a time for imaging to a time obtained by dividing the time for the imaging by the number of the sub coils.
In order to realize the parallel imaging, it is necessary that an electromagnetic coupling between the sub coils be sufficiently small. When the electromagnetic coupling is present between the sub coils, noise interferes with an image signal at a location between the coils to reduce a signal-to-noise (S/N) ratio of an image. In addition, it is necessary that the plurality of sub coils be appropriately arranged. When the sub coils are not appropriately arranged, the S/N ratio of the image is partially reduced. There is a standard (a formula for calculation of the standard is described in Non-Patent Document 2) called a Geometry factor (hereinafter referred to as a G factor) as a standard used to evaluate whether or not the sub coils are appropriately arranged. The G factor is a numerical value of 1.0 or more and obtained from a distribution of sensitivity (to a cross section of a subject to be imaged) of each of the sub coils. The S/N ratio of each region of the image is proportional to 1/G (factor). It is, therefore, desired that a G factor for a region in which a subject to be imaged is present be as small as possible. In general, it is desired that the G factor be smaller than 2.0. In order to design a receiving coil for the parallel imaging, it is necessary to reduce the electromagnetic coupling between the plurality of sub coils used for a simultaneous signal measurement and find a coil arrangement ensuring that G factors are small for all cross sections (of a subject) to be imaged. The parallel imaging has been developed by using a horizontal magnetic field apparatus that generates a high magnetic field. Various types of receiving coils for horizontal magnetic field apparatus have been proposed.
On the other hand, a vertical magnetic field open MRI apparatus is excellent for openness of a magnet and is therefore capable of providing direct access to a subject to be imaged. The vertical magnetic field open MRI apparatus is suitable for interventional MRI. It is necessary that the direction of an RF magnetic field generated by an RF coil be perpendicular to the direction of a static magnetic field. Therefore, when the direction of the static magnetic field is changed from a horizontal direction to a vertical direction, it is also necessary that the configuration of a receiving coil be changed. In the vertical magnetic field type MRI apparatus, the direction of a static magnetic field is parallel to the vertical direction. Thus, a solenoid coil can be used. In this case, the solenoid coil is arranged around a subject that is to be imaged and normally lies down in the horizontal direction. The solenoid coil arranged around the subject to be imaged is different from a loop coil placed above the surface of the subject to be imaged, and has high sensitivity to a deep portion of the subject. When the intensity of a magnetic field generated by the vertical magnetic field type MRI apparatus is the same as that of a magnetic field generated by a horizontal magnetic field type MRI apparatus, the vertical magnetic field type MRI apparatus capable of using a solenoid coil generally has higher sensitivity to a deep portion of a subject to be imaged than that of the horizontal magnetic field type MRI apparatus.
An arrangement of a receiving coil for a vertical magnetic field is disclosed, for example, in each of Patent Documents 1 and 2. Patent Document 1 discloses that a combination of a plurality of solenoid coils arranged around a subject to be imaged with a plurality of surface coils is used as a receiving coil for vertical magnetic field type MRI; and parallel imaging is performed on a region near the heart located at a deep portion of the subject to be imaged to image the region with high sensitivity and at high speed. In Patent Document 2, a solenoid coil and a saddle coil, which are perpendicular to each other, are used to improve sensitivity to a deep portion of a subject to be imaged; and at least two sub coils are arranged in each of three directions with respect to a subject to be imaged. Since the at least two sub coils arranged in each of the three directions face each other, sensitivity profiles of the sub coils are created for three phase encoding directions. When the receiving coil arranged in the abovementioned way is used, the receiving coil has high sensitivity to a deep portion of a subject to be imaged, and high speed imaging is possible regardless of a selected phase encoding direction.
Non-Patent Document 1: J. B. Ra, C. Y. Rim: “Fast Imaging Using Subencoding Data Sets from Multiple Detectors”, Magnetic Resonance in Medicine, vol. 30, pp. 142-145 (1993)
Non-Patent Document 2: Klaas P. Pruessmann, Markus Weiger, Markus B. Scheidegger, and Peter Boesiger: “SENSE: Sensitivity Encoding for Fast MRI”, Magnetic Resonance in Medicine, vol. 42, pp. 952-962 (1999).
Patent Document 1: JP-A-2002-153440
Patent Document 2: JP-A-2003-79595